This invention relates to a device for measuring the ferrite content in a weld deposit of an austenitic stainless steel weld material. A small but controlled amount of ferrite is effective in preventing cracking of austenitic stainless steel weld deposits and therefore it is important to know, as accurately as possible, the exact amount of ferrite present in the weld joint deposit. An incorrect amount of ferrite in the weld deposit leads to cracking and deterioration of the welded joint. Types of ferrite-containing austenitic weld metals which take advantage of the effect of ferrite include among others the 308, 308L, 309, 347, and 318 electrode or rod types. In cooling from the molten state austenitic stainless weld metal of normal carbon content solidifies first as a mixture of ferrite and austenite; most of the ferrite subsequently transforms to austenite as the deposit cools. The two factors which have the greatest affect on the ferrite content of the weld deposit are the chemistry of the weld electrode or rod and the welding technique itself.
Heretofore, the amount of ferrite in stainless steel weld deposits has been determined in any of several ways. Metallographic examination of the deposit can provide an approximate indication of the ferrite content in terms of area distribution. However, such a method is cumbersome and requires considerable care to be sure that the sections examined give a true picture of the volumetric ferrite distribution. Various diagram methods have been used for calculating the amount of ferrite in weld deposits based on chemical analysis by graphically combining effects of the austenitizers nickel, carbon, and manganese, and the ferritizers chromium, molybdenum, silicon, and columbium. However, the accuracy of such calculations depends on how accurately the chemistry of the weld deposit is known. Some of the more well known diagram methods include the Schaeffler and the DeLong diagrams.
Prior art devices have been developed which use a magnet suspended from a beam and associated strain gauges for measuring the thickness of a material, such as U.S. Pat. No. 3,244,971. However, such devices are rather complicated as they are used for measuring flexing in two directions and would not provide for the precision required for measuring very small amounts of ferrite. Other prior art devices such as shown in U.S. Pat. No. 3,745,449 use electronic circuitry for measuring uniform wall thickness in the welded joints of tubular pipe by detecting magnetic flux variations in the weld line, but such a device would not be suitable for accurately measuring small amounts of ferrite.